KcsA Potassium channel
Streptomyces lividans (bacteria)
In organisms consisting of multiple cells, they must exchange information to act cooperatively. The exchange of information is in part accomplished by the ions, such as sodium, potassium and calcium. The information is transmitted by ions permeating through the ion channel of the cell membrane. For solving why only certain ion is admitted through each ion channel by studying the function and 3D structure of the ion channels, the Nobel Prize in Chemistry for 2003 was given.
The potassium channel from Streptomyces lividans is a membrane protein, and the length of the pore of this channel is 45 angstrom, which consists of tunnel, cavity and filter as described below. From inside the cell, the pore begins as a tunnel 18 angstrom in length. A molecular sensor controls the opening and closing of the entrance of pore, and depending on the difference between the concentrations of potassium ions inside and outside the cell membrane, the gate either opens or closes. When the gate is opened, potassium ions surrounded with some water molecules move to the cavity at the center of cell membrane. As this cavity is negatively charged and water molecules can reach here as well, positively charged potassium ions are stabilized even in the low dielectric membrane center. There is a 12 angstrom-long filter between the cavity and outside the cell membrane, which only potassium ions but not water molecules and other ions such as sodium are admitted to pass through. This selectivity is achieved because the filter is designed so that only a dehydrated potassium ion can fit. The potassium ions at close proximity within the filter repel each other by electrostatic forces. Thus, the potassium ions diffuse across cell membranes through the potassium channel with complicated system. The potassium ions play an essential role in the nervous system and muscle, which makes the ion channels important drug targets and this structure important.
Protein Data Bank (PDB)
Doyle, D.A. Morais Cabral, J. Pfuetzner, R.A. Kuo, A. Gulbis, J.M. Cohen, S.L. Chait, B.T. MacKinnon, R.; "The structure of the potassium channel: molecular basis of K+ conduction and selectivity."; Science; (1998) 280:69-77 PubMed:9525859.
author: Yuko Tsuchiya